U.S. patent number 6,891,870 [Application Number 10/037,461] was granted by the patent office on 2005-05-10 for distributed feedback laser for isolator-free operation.
This patent grant is currently assigned to Corning Lasertron, Inc.. Invention is credited to Angela Hohl-Abichedid, Hanh Lu, Richard T. Sahara.
United States Patent |
6,891,870 |
Sahara , et al. |
May 10, 2005 |
Distributed feedback laser for isolator-free operation
Abstract
An integrated semiconductor device comprising a laser on a
substrate, the laser having an active layer and a current-induced
grating, such as a current-injection complex-coupled grating,
within a laser cavity producing a single-mode output light signal
at high data rates (>622 Mb/sec) in isolator-free operation. The
grating has a coupling strength product .kappa.L greater than 3,
where .kappa. is the coupling coefficient and L is the length of
the laser cavity. In certain embodiments, the laser is a
distributed feedback (DFB) laser that emits light at a wavelength
of about 1.5 .mu.m. The strong current-induced grating prevents
mode hopping between multiple degenerate Bragg modes. The laser is
also characterized by excellent immunity from optical feedback, and
can be operated without an isolator at high data rates.
Inventors: |
Sahara; Richard T. (Watertown,
MA), Hohl-Abichedid; Angela (Beverly, MA), Lu; Hanh
(North Andover, MA) |
Assignee: |
Corning Lasertron, Inc.
(Bedford, MA)
|
Family
ID: |
21894473 |
Appl.
No.: |
10/037,461 |
Filed: |
November 9, 2001 |
Current U.S.
Class: |
372/46.01 |
Current CPC
Class: |
B82Y
20/00 (20130101); H01S 5/1228 (20130101); H01S
5/34373 (20130101); H01S 5/3406 (20130101) |
Current International
Class: |
H01S
5/00 (20060101); H01S 5/12 (20060101); H01S
5/343 (20060101); H01S 5/34 (20060101); H01S
003/08 () |
Field of
Search: |
;372/43,46 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Huang et al., Isolator-Free 2.5-Gb/s 80-km Transmission by Directly
Modulated Iamda/8 Phase-Shifted DFB-LDs Under Negative Feedback
Effect of Mirror Loss, Mar. 2001, IEEE Photonics Technology
Letters, vol.13, pp. 245-247.* .
Kazmierski, Christophe, et al., "1.5.mu.m DFB Lasers with New
Current-Induced Gain Gratings," IEEE Journal of Selected Topics in
Quantum Elec., 1(2): 371-374 (1995). .
Nakano, Yoshiaki, et al., " Reduction of Excess Intensity Noise
Induced by External Reflection in a Gain-Coupled Distributed
Feedback Semiconductor Laser," IEEE Journal of Quantum Electronics,
27(6): 1732-1735 (1991). .
Huang, Yidong,et al., "Isolator-Free 2.5 Gb/s 80-km Transmission by
Directly Modulated .lambda./8 Phase-Shifted DFB-LDs Under Negative
Feedback Effect of Mirror Loss," IEEE Photonics Technology Letters,
13(3): 245-247 (2001). .
Thedrez, B., et al., "1.3.mu.m tapered DFB lasers for isolator-free
2.5 Gbits all-optical networks," OPTO+, Groupement d'Interet
Economique, Alcatel Corporate Research Center, Marcoussis, France.
.
Xing-sha, Zhou and Peida, Ye, "Intensity Noise of Semiconductor
Laser In Presence Of Arbitrary Optical Feedback," Electronics
Letters, 25(7): 446-447 (1989). .
Schunk, N. and Petermann, K., " Measured Feedback-induced Intensity
Noise for 1.3 .mu.m DFB Laser Diodes, " Electronics Letters, 25(1):
63-64 (1989). .
Favre, F., "Sensitivity to External Feedback For Gain-Coupled DFB
Semiconductor Lasers," Electronics Letters, 27(5): 433-435 (1991).
.
Nakano, Y., et al., "Resistance to External Optical Feedback in a
Gain-Coupled Semiconductor DFB Laser," University of Tokyo,
Bunkyo-ku, Tokyo 113, Japan. .
"QLM6S891, 2mW 1625nm OSC Source DFB Laser", Product Brochure,
Corning Incorporated, One Riverfront Plaza, Corning, NY 14831-0001
(2001)..
|
Primary Examiner: Wong; Don
Assistant Examiner: Nguyen; Dung (Michael) T
Attorney, Agent or Firm: Agon; Juliana
Parent Case Text
RELATED APPLICATION
This application is related to U.S. patent application Ser. No.
10/037,458 entitled TUNABLE LASER DEVICE FOR AVOIDING OPTICAL MODE
HOPS, by Richard Sahara, Randal Salvatore, and Hanh Lu, filed on
even date herewith and commonly owned by the owner of this
application. The entire teachings of the above application are
incorporated herein by reference.
Claims
What is claimed is:
1. An integrated semiconductor device comprising: a semiconductor
substrate; a laser on the substrate having an active layer and a
periodically spaced current-induced grating disposed near the
active layer, wherein the periodically spaced current-induced
grating modulates gain in the active layer in the direction of
light propagation for providing periodic modulation of the gain of
the active layer and periodic modulation of a differential
refractive index between the different indices of the active layer
and of the periodically spaced current-induced grating to determine
a wavelength of a light emitted from a laser cavity formed from the
length L of the active layer, wherein the light emitted is a
single-mode output light signal at a data rate greater than 622
Mb/sec in isolator-free operation; and an electrical contact over
the periodically spaced current-induced grating for providing
current to the grating to control the wavelength of the light
emitted from the laser.
2. The semiconductor device of claim 1 wherein the grating
comprises a strong complex-coupled grating having a coupling
strength producing .kappa.L greater than 3, where .kappa. is a
coupling coefficient.
3. The semiconductor device of claim 2 wherein the grating
comprises a first semiconductor material overgrown with a second
semiconductor material.
4. The semiconductor device of claim 1 wherein the active layer
comprises a multiple quantum well structure.
5. The semiconductor device of claim 4 wherein the multiple quantum
well structure is AlInGaAs.
6. The semiconductor device of claim 1 wherein the electrical
contact provides current to the grating at the data rate of at
least 2.5 Gb/sec.
7. The semiconductor device of claim 1 further comprising a
modulator on the substrate for modulating the output light.
8. The semiconductor device of claim 7 wherein the modulator
comprises an electroabsorption modulator.
9. The semiconductor device of claim 7 wherein the modulator
comprises a Mach Zehnder modulator.
10. The semiconductor device of claim 1 wherein the laser comprises
a distributed feedback (DFB) laser.
11. A method for fabricating an integrated semiconductor device
comprising: forming on a semiconductor substrate an active layer;
and forming a periodically spaced current-induced grating above the
active layer, wherein the periodically spaced current-induced
grating modulates gain in the active layer in the direction of
light propagation for providing periodic modulation of the gain of
the active layer and periodic modulation of a differential
refractive index between the different indices of the active layer
and of the periodically spaced current-induced grating to determine
a wavelength of a light emitted from a laser cavity formed from the
length L of the active layer, wherein the light emitted is a
single-mode output light signal at a data rate greater than 622
Mb/sec. in isolator-free operation; and forming an electrical
contact over the periodically spaced current-induced grating for
providing current to the grating to control the wavelength of the
light emitted from the laser.
12. The method of claim 11 wherein the output light has a
wavelength of about 1.5 .mu.m.
13. The method of claim 11 wherein the grating comprises a strong
complex-coupled grating having a coupling strength product .kappa.L
greater than 3, where .kappa. is a coupling coefficient.
14. The method of claim 11 wherein the grating comprises a first
semiconductor material overgrown with a second semiconductor
material.
15. The method of claim 11 wherein the active layer comprises a
multiple quantum well structure.
16. The method of claim 11 wherein the multiple quantum well
structure is AlInGaAs.
17. The method of claim 11 further comprising forming a modulator
on the substrate for modulating the output light.
18. The method of claim 17 wherein the modulator comprises an
electroabsorption modulator.
19. The method of claim 17 wherein the modulator comprises a Mach
Zehnder modulator.
20. An optical communication device comprising: a semiconductor
laser having an active layer and a periodically spaced
current-induced grating disposed near the active layer, wherein the
periodically spaced current-induced grating modulates pain in the
active layer in the direction of light propagation for providing
periodic modulation of the gain of the active layer and periodic
modulation of a differential refractive index between the different
indices of the active layer and of the periodically spaced
current-induced grating to determine a wavelength of an output
light emitted from a laser cavity formed from the length L of the
active layer, wherein the output light is a single-mode output
light signal at a data rate greater than 622 Mb/sec. an electrical
contact over the periodically spaced current-induced grating for
providing current to the grating to control the wavelength of the
output light emitted from the laser; an optical fiber for receiving
the output light; and optics for isolator-free coupling of the
output light into the optical fiber.
21. The device of claim 20 wherein the output light has a
wavelength of about 1.5 .mu.m.
22. The device of claim 20 wherein the grating comprises a strong
complex-coupled grating having a coupling strength product .kappa.L
greater than 3, where .kappa. is a coupling coefficient.
23. The device of claim 22 wherein the grating comprises a first
semiconductor material overgrown with a second semiconductor
material.
24. The device of claim 20 wherein the active layer comprises a
multiple quantum well structure.
25. The device of claim 24 wherein the multiple quantum well
structure is AlInGaAs.
26. The device of claim 20 wherein the electrical contact provides
current to the grating at the data rate of at least 2.5 Gb/sec.
27. The device of claim 20 further comprising a modulator
integrated with the laser for modulating the output laser light
before coupling into the optical fiber.
28. The device of claim 27 wherein the modulator comprises an
electroabsorption modulator.
29. The device of claim 27 wherein the modulator comprises a Mach
Zehnder modulator.
30. The device of claim 20 wherein the laser comprises a
distributed feedback (DFB) laser.
31. The device of claim 20 wherein the optics for isolator-free
coupling comprise at least one lens disposed between the laser and
the optical fiber.
32. The device of claim 31 wherein the optics for isolator-free
coupling comprise at least two lenses disposed between the laser
and the optical fiber, including a collimating lens and a coupling
lens.
33. The device of claim 31, wherein the at least one lens comprises
a fiber lens at an end of the fiber for receiving the output light.
Description
BACKGROUND OF THE INVENTION
Semiconductor laser devices are widely used in fiber optic
communication systems. Distributed feedback (DFB) lasers have
proven particularly successful in achieving narrow-linewidth,
single-mode light for high-speed data transmission applications.
The DFB device employs a Bragg grating structure for providing
improved selectivity in the output light mode. The grating
structure introduces a periodic modulation of the refractive index
or gain within the laser cavity, thus facilitating single
longitudinal mode oscillation within the cavity.
For low-error performance, conventional DFB lasers require optical
isolation to prevent optical feedback, including external
reflections, from coupling back into the laser chip. In a
conventional DFB laser, the grating structure promotes the
formation of two standing waves in the cavity (180.degree. out of
phase with each other), often referred to as the degenerate Bragg
modes. In order to achieve single-mode, single-wavelength emission,
the degeneracy must be "broken" so that only one of the Bragg modes
will be pumped. Typically, the degeneracy is broken by reflections
from the facets of the laser cavity, which strengthen one mode
relative to the other, in essence "selecting" a single mode to
lase. However, reflections from sources external to the chip may
serve to break the degeneracy of the Bragg modes in an undesirable
way, causing the laser output light to jump intermittently between
multiple Bragg modes. Optical isolators are therefore required in
order to maintain the single-mode, single-wavelength output light
desired for optical communication. The need to shield the device
from external optical feedback is particularly acute at high-data
rates where dispersion effects may render long-span transmission
impossible.
Typical optical isolators for use with DFB lasers include a pair of
polarizers with an intervening Faraday rotator, or a
polarization-maintaining pigtailed optical isolator, which is a
passive device that allows light to travel through a fiber in one
direction only. These isolators add significant cost to fiber optic
communication systems, and complicate the construction of
multifunction integrated devices, such as dense WDM systems
employing compact multichannel, multiwavelength sources.
SUMMARY OF THE INVENTION
An integrated semiconductor device of the present invention
comprises a laser on a substrate, the laser having an active layer
and a current-induced grating, such as a current-injection
complex-coupled grating, within a laser cavity producing a
single-mode output light signal at high data rates (>622 Mb/sec)
in isolator-free operation. The grating has a coupling strength
product .kappa.L greater than 3, where .kappa. is the coupling
coefficient and L is the length of the laser cavity. In certain
embodiments, the laser is a distributed feedback (DFB) laser that
emits light at a wavelength of about 1.5 .mu.m, or in other cases,
at a wavelength about 1.3 .mu.m.
The coupling strength product, .kappa.L, is a measurement of the
grating strength in the DFB laser, where .kappa. (cm.sup.-1) is a
coupling coefficient relating to the extent that light is coupled
backward over the distributed length of the cavity, and L (cm) is
the length of the cavity. Conventionally, .kappa.L products between
0.5 and 1.5 are preferred for most DFB lasers, as weaker values
will not provide the selectivity for single-mode output, and
stronger values, it is believed, cause the DFB to lase in multiple
degenerate Bragg modes.
According to one aspect, high-yield semiconductor laser devices
with large .kappa.L products (e.g. >3) are provided that use a
complex-coupled grating structure. In this device, the grating
material introduces a difference in the index of refraction of the
laser material. The grating structure is also located just above
the active region, causing the injection of current, and therefore
the gain in the active region to be modulated spatially in the
direction of light propagation. The longer wavelength of the two
Bragg modes is selected by the current-induced gain modulation, and
not by the reflection of the cleaved facets. The grating coupling
strength is free to be made very strong relative to reflection from
the cleaved facets and external sources.
The laser device of the present invention is further characterized
by excellent immunity from external optical feedback, which permits
the laser to be operated in a single longitudinal mode at high data
rates without the use of complicated and costly optical isolators.
More particularly, the device of the present invention manages to
eliminate the problem of mode hopping between degenerate Bragg
modes without resorting to conventional optical isolation schemes.
The removal of the optical isolator is not only a significant cost
reduction, it also simplifies the construction of many
multifunction integrated devices.
The integrated semiconductor device of the invention may include a
directly-modulated laser, or a laser coupled with an external
modulator for forming the optical signals. For instance, the device
may comprise a laser and an electroabsorption modulator (EML), or a
Mach Zehnder modulator, integrated on a single substrate.
The invention also relates to a method for fabricating an
integrated semiconductor device, comprising forming an active layer
above a substrate, and forming a current-induced grating above the
active layer, so as to produce a laser cavity emitting a
single-mode output light signal at a high data rate in
isolator-free operation.
The invention further relates to an optical communication device
comprising a semiconductor laser having an active layer and a
current-induced grating that form a laser cavity producing a
single-mode output light signal at a high data rate, an optical
fiber for receiving the output light signal, and optics for
isolator-free coupling of the output light into the optical fiber.
The coupling optics can include, for instance, a collimating lens
for collimating the light emitted by the laser and a coupling lens
for focusing the light into the optical fiber. In other
embodiments, a fiber pigtail arrangement may be employed, where the
optical fiber is connected to the laser device at the output facet,
and the coupling optics includes an endface lens at the connecting
end of the optical fiber.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram showing an optical communication device
according to the present invention;
FIG. 2 is a cross-sectional view of a distributed feedback (DFB)
laser device with a current-induced grating according to the
present invention;
FIGS. 3A, 3B to 5A, 5B illustrate side and front facet views,
respectively, of process steps for fabricating a DFB laser device
of the present invention;
FIG. 6 is a schematic block diagram of a test system for DFB laser
modules in isolator-free operation;
FIG. 7 shows power versus intensity curves for a first DFB module
in isolator-free operation at different optical feedback
levels;
FIG. 8 shows power versus intensity curves for a second DFB module
in isolator-free operation at different optical feedback
levels;
FIG. 9 shows the output spectrum for a DFB module in isolator-free
operation at two optical feedback levels;
FIG. 10 shows the side mode suppression ratio (SMSR) for DFB
modules in isolator-free operation as a function of optical
feedback;
FIG. 11 shows the spectrum for a DFB module in isolator-free
operation measured with an interferometer;
FIG. 12 shows the relative intensity noise (RIN) spectra for a DFB
module in isolator-free operation at different optical feedback
levels;
FIG. 13 shows the RIN for DFB modules in isolator-free operation at
1.8 Ghz plotted as a function of the optical feedback into the
module;
FIG. 14 shows bit error rate (BER) curves for a DFB module in
isolator-free operation;
FIG. 15 shows dispersion penalty for DFB modules as a function of
optical feedback into the module.
The foregoing and other objects, features and advantages of the
invention will be apparent from the following more particular
description of preferred embodiments of the invention, as
illustrated in the accompanying drawings in which like reference
characters refer to the same parts throughout the different views.
The drawings are not necessarily to scale, emphasis instead being
placed upon illustrating the principles of the invention.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 is a block diagram of an optical communication device 100,
which has been constructed according to the principles of the
present invention. As is common in these devices, a semiconductor
laser 110 generates an optical signal 112. In one embodiment, the
laser is a distributed feedback laser (DFB) that emits light having
a wavelength of approximately 1550 nm.
Information is encoded into the optical signal 112 by modulating
the laser's injection current in response to an information signal
118. Specifically, an injection current controller 116, typically
external to the communications device 100, receives the information
signal 118 from the device 100. The controller 116 modulates the
injection current of the laser 110 both to encode the information
signal and maintain nominal, predetermined output optical powers to
conform with both the device's damage thresholds and user design
specifications.
A thermo-electric cooler 128 maybe provided for maintaining a
constant, controlled operating temperature, and thus a stable
wavelength for the optical signal 112. The thermo-electric cooler
is under the control of temperature control circuitry 131, which
modulates the operation of the thermo-electric cooler 128 to heat
or cool the laser in response to temperature signals received from
thermistor 130.
In addition to the directly-modulated DFB laser embodiment
described above, the invention also applies to other modulated
laser light systems. For example, in alternative embodiments, the
laser can be replaced with a laser diode and discrete modulator,
such as an electroabsoption modulator or a Mach Zehnder modulator.
Typically, in such systems, the laser runs nominally in a
continuous wave mode with the information signal 118 being provided
to the modulator.
Exemplary embodiments of an integrated electroabsorption modulated
laser (EML) device, as well as an electroabsorption modulated
partial grating laser (EMPGL) device, wherein the laser diode and
modulator are integrated on a single substrate, are disclosed in
commonly-owned U.S. patent application Ser. No. 09/809,725,
entitled "ELECTROABSORPTION MODULATED LASER," by Randal A.
Salvatore, Richard P. Sahara and Hanh Lu, the entire teachings of
which are incorporated herein by reference.
In FIG. 1, the optical signal 112 is transmitted via an optical
fiber 10 that is external to the communications device 100.
Collimating lens 122 and coupling lens 126 are used to couple the
optical signal 112 into the optical fiber for transmission in a
typical embodiment.
According to one aspect of the present invention, the
communications device 100 operates at a high rate of data
transmission (>622 Mb/sec) without the use of an optical
isolator. In conventional optical communication systems, an optical
isolator is disposed between the output facet 114 of the laser 110
and the optical fiber 10, typically between the collimating lens
122 and coupling lens 126. The optical isolator is required to
prevent undesirable optical feedback, including unintended
reflections, from entering the laser chip and interfering with
device performance. In the present invention, however, there is no
optical isolator, and the output light from the laser device is
directly coupled by lenses 122, 126 into the optical fiber.
In other embodiments, alternative isolator-free optical coupling
systems can be employed. For instance, a fiber-pigtail can be
connected to the output facet of the laser, and the output light
can be directly coupled into the optical fiber by an endface
lens.
Turning now to FIG. 2, an exemplary embodiment of a complex-coupled
ridge waveguide DFB laser device 200 of the present invention is
illustrated. FIG. 2 shows a cross-sectional view of the laser
device 200 taken along the length of the ridge waveguide. The
device is constructed on a substrate 16, which, according to this
embodiment, comprises n+-type indium phosphide (InP). Above the
substrate 16 is buffer layer 30 that is n+-type InP.
In succession above the buffer layer 30 is a lower cladding layer
32, a lower carrier confinement layer 34, an active layer 18, an
upper carrier confinement layer 36 and an upper cladding layer 38.
The lower and upper cladding layers 32, 38 provide optical
confinement and are preferably of InP of n-type and p-type,
respectively. The lower and upper carrier confinement layers 34, 36
confine the holes and electrons in the active layer 18 and are
preferably of aluminum indium gallium arsenide (AlInGaAs). The
active layer 18 is a multiple quantum well (MQW) structure which is
also of AlInGaAs. The MWQ structure includes a succession of wells
and potential barriers ranging from 4 to 20, but preferably about
5. The strain of the wells and barrier materials is compensated to
improve the gain, carrier injection efficiency, and material
reliability.
Alternative materials for forming the active layer 18 include
InGaAsP, InGaAs, and AlGaNAs.
A periodic complex-coupled Bragg grating structure 20 is formed in
the upper cladding layer 38. The Bragg grating is a three level
structure of InP/InGaAsP/InP, periodically spaced along the length
of the cavity. In this example, the periodic spacing is
.lambda./2n, where .lambda. is the emission wavelength of the laser
and n is the effective index of refraction of the optical mode.
The periodic Bragg grating is a complex coupled grating because the
grating structure provides periodic modulation of both refractive
index and gain within the laser cavity. The refractive index
modulation is introduced by the selection of the narrow band gap
(e.g. 1.1 .mu.m) grating material 20, having a different index than
the surrounding material 38. The gain modulation results from the
difference in conductivity, obtained by dopant or band gap, between
InGaAsP grating structure and the surrounding InP cladding layer.
As this grating structure is located just above the active layer
18, current injection causes the gain of the active layer to be
modulated spatially in the direction of light propagation. This
selectivity of current flow causes only one of the degenerate Bragg
modes to be pumped. In the embodiment shown in FIG. 2, the spacing
52 between the grating and upper carrier confinement layer is on
the order of 0.1 .mu.m to provide the current-injection
complex-coupling.
The resulting layered structure of the laser 200 provides a DFB
laser that confines light generated in the active layer 18
primarily within a resonant cavity including the active, carrier
confinement, Bragg grating and cladding layers. An upper etch-stop
layer 40 of InGaAsP, formed in the upper cladding layer 38, is used
to define the depth of the ridge waveguide.
Above the upper cladding layer 38 is provided in succession a band
gap transition layer 42 of InGaAsP, an electrical contact layer 44
of p+-type InGaAs, and metal contact 46. In operation, contact 46
is forward-biased.
Generally, the current-induced grating in the present invention is
significantly stronger than in conventional DFB devices,
particularly index-coupled devices. The strong grating provides
robust, single-mode output, while also preventing external feedback
from breaking the degeneracy of the Bragg modes in an undesirable
way. Depending on the application, the .kappa.L product for the
grating may be greater than 3 or 4, or even larger. A
complex-coupled DFB with .kappa.L=7, for example, should show two
orders better immunity to stray reflections than the more typical
DFB with .kappa.L=1.5. It is also possible to have relatively high
output power even with a very strong grating (e.g. .kappa.L
.about.6 or 7) and where the facet output power is only a fraction
of the power in the center of the laser. In practical terms, the
grating strength is limited by its construction of the output slope
efficiency. This limit is similar to the low slope efficiency of
Fabry Perot lasers with very high reflective facets.
Current-induced complex-coupled DFB lasers, such as the device of
FIG. 2, are also advantageous with respect to many gain-coupled
devices in that the grating structure of the present invention is
fabricated above the active layer 18, without requiring an etch
into the active layer, or another process which might detrimentally
affect the active portion of the laser. In the DFB of FIG. 2, for
example, the aluminum in the AlInGaAs active region is extremely
reactive to air, and an etching process into the active region
introduces a great risk of contamination of this sensitive
structure. According to the present invention, the grating may be
formed above the active layer 18, while the thin layer of InP 52
provides protection for the active region. Many gain-coupled
devices, by contrast, utilize an etched or corrugated active layer
for providing the periodic gain modulation, where the active region
cannot be similarly protected during the grating fabrication
process.
Process steps for fabricating the complex-coupled DFB laser device
of FIG. 2 are now described. Reference is made to FIGS. 3A, 3B to
5A, 5B in the following description of process steps wherein side
(A) and front (B) facet views are provided.
As shown in FIGS. 3A, 3B, an InP substrate and buffer layer,
jointly designated 18A, lower cladding layer 32, active region
including upper and lower carrier confinement layers and active
layer, jointly designated 18A, and upper cladding layer 38 are
formed using an epitaxial growth process, such as metalorganic
chemical vapor deposition (MOCVD), using material compositions
described above with reference to FIG. 2. A quaternary (InGaAsP)
1.1 .mu.m layer 40A for the grating structure is formed on the
upper cladding layer, followed by a thin layer of InP.
The grating is fabricated by depositing a photoresist, and exposing
the photoresist to a holographic pattern to provide spaced portions
20A for producing the grating. Following this step, a timed wet
etch is used to make the grating, and the photoresist is then
stripped.
As shown in FIGS. 4A, 4B, overgrowth begins with InP 38 followed by
an upper etch-stop layer 40 which is a quarternary (InGaAsP) 1.3
.mu.m layer above the grating 20, followed by an additional layer
of InP 38, a transition layer 42 of InGaAsP, and electrical contact
layer 44 of p+-type InGaAs. Above electrical contact layer 44 may
be grown a protection layer 47 of InP.
The ridge waveguide is defined by first removing the protection
layer 47 and depositing an SiO.sub.2 -A layer and photoresist. The
photoresist is exposed with a ridge mask followed by development
and stripping of the photoresist. Using a dry etch (CH.sub.4
/H.sub.2 /Ar), the photoresist pattern is etched down into the
upper cladding layer 38. A wet etch using HCl:H.sub.3 PO.sub.4
(1:3) is then done down through upper etch-stop layer 40 to further
define the ridge 22A.
P-metals (e.g., Ti 400 .ANG./Pt 1000 .ANG./Au 2500 .ANG.) are
deposited over InGaAs contact layer 44. An electroplate mask can be
used to plate about 1.5 .mu.m Au.
The device is wafer-thinned to facilitate cleaving, and then
receives Au/Sn n-contact metal. The device is then annealed to
410.degree. C., is cleaved into bars and is facet coated.
Experimental results for complex-coupled DFB laser devices in
isolator-free operation in accordance with the present invention
are now described. More particularly, the performance of 1550 nm
DFB laser diode modules were examined and characterized with up to
20% back reflection into the modules. The lasers included
current-induced complex-coupled grating structures having a strong
coupling coefficient of .kappa.=51 cm.sup.-1. In general, through
bit error rates 10.sup.-10, no error floor was found and the
dispersion penalty remained less than 2.0 dB for a data rate of 2.5
Gb/sec. over 100 km of SMF-28.
The subject devices were fabricated in accordance with the process
steps described above. In particular, four ridge waveguide laser
devices ridge were formed with standard dry and wet processes,
cleaved and given a 2% AR coating on the front facet and left as
cleaved on the back facet. Modules 1, 2 and 3 were first assembled
with an optical train consisting of the chip, lens, isolator, lens
and angle cleaved fiber. After initial characterization, the
isolators were removed, and the chips were tested in detail with
the remainder of the optical path intact. Module 4 was assembled
with light from the chip coupled directly into a conical tip,
lensed fiber. For both configurations, the coupling efficiencies of
light into the single mode optical fiber was approximately 60%.
A schematic of the test system is shown in FIG. 6, including DFB
module 300, DC bias source 301, bit error rate pattern generator
302, power monitor 303, power meter 304, optical spectrum analyzer
305, fiber interferometer 306, relative intensity noise detector
307, bit error rate detector 308, variable optical attenuator 309,
polarization controller 310, and gold tip fiber 311. The back
reflection through the fiber power splitters 312, variable optical
attenuator 309, polarization controller and the gold tipped fiber
311 was carefully calibrated. In this context, the back reflection
is referenced relative to the module fiber power rather than the
chip facet power. The fiber polarization controller 310 was
adjusted so that the polarization of the feedback was parallel to
the TE mode which the modules normally emit so as to maximize the
interaction of the reflected light with the laser at threshold. The
distance from the laser to the back reflection source is
approximately 8.7 meters in this system. Angled connectors were
used to avoid unintended back reflections. With no back
reflections, the CW linewidth of these devices is much less than 1
MHz at 1 mW fiber power, giving a coherence length in optical fiber
of better than 200 meters. Therefore, the reflection source is
within the coherence length of the original isolated laser module.
Back reflections up to 20% were studied since this corresponds to
the maximum reflection from an air gap at a connector.
It can be seen from FIG. 7 that no kinks were found in the power
versus current curves for Module 4. The curves with -36.5, -15,
-10.2, -8.6 and -7.0 dB of feedback are offset vertically in 1 mW
increments for clarity. The threshold current of 24.1 mA was
unchanged from -36.5 dB to -22 dB feedback, while it decreased 1.2
mA as the feedback was increased from -22 dB to -7.0 dB. FIG. 8
shows the power versus current curve for Module 1 with significant
kinks for the curves with large amounts of back reflection. The
kink is consistent with the transition from a single external
cavity longitudinal mode to a regime with a multiple external
cavity modes. In spite of the kink, the dispersion penalty
performance with high back reflection was still surprisingly good
as presented below.
The spectrum of Module 4 in FIG. 9 shows little change with optical
feedback. This was observed in all the devices as indicated in FIG.
10 plotting the side mode suppression ratio (SMSR) at 1 mW as a
function of back reflected power. Due to the strong complex coupled
grating, the devices always operated with an SMSR well above 45 dB.
The spectrum was carefully studied near the discontinuities in the
P-I curve and various effects were ruled out. Because of the
difference in effective refractive index between the lowest and
second lowest transverse modes, the second lowest transverse mode
would be expected in the spectrum about 6 nm shorter in wavelength
than the main peak, but this was not observed. The complex coupled
gratings kept the devices locked on the long wavelength side of the
1.2 nm stop band as shown in FIG. 9. This stop band corresponds to
a grating strength of 51 cm.sup.-1 giving .kappa.L=3.1. Also, no
mode hopping between the two Bragg modes was observed. The main
peak was not pulled between Fabry Perot modes spaced 0.6 nm apart.
Although the submount temperature was set at 25.degree. C., the
center wavelength shifted evenly at a rate of 0.00585 nm/mA from
localized heating in the chip relative to the monitoring
thermistor. From threshold current to 100 mA, the localized heating
resulted in a gradual 0.47 nm peak wavelength shift. Mode hopping
between external cavity modes separated by 0.000104 nm (11.4 MHz)
was beyond the 0.1 nm resolution limit of the optical spectrum
analyzer. However, the 0.47 mn of tuning from the localized heating
would tune across some 4500 external cavity modes rather than the
single discontinuity observed in the P-I curve.
To examine the fine features in the CW spectrum at the
discontinuity in the P-I curve, Module 1 was characterized by the
self-homodyne method. The 5.2 km (26 microsecond) optical fiber
delay path gives a 38 kHz resolution limit for the incoherent
regime of interferometry. The three curves in FIG. 11 are offset by
30 dB for clarity. For the bottom curve, the laser was operated at
1 mW fiber power (34 mA) without an intentional back reflection.
The device operated in a single longitudinal mode. For the middle
trace, the laser was also operated at 1 mW, but with -7.0 dB back
reflection, which corresponds to the stable region of the P-I
curve. Under these conditions, external cavity modes at 11.4 MHz
and 22.8 MHz are quite small relative to the main peak. Additional
external cavity modes in the 50-300 MHz regime are also well
suppressed. Because of the time averaging nature of the
measurement, it was not clear if the side modes operate
simultaneously with the main mode or represent transitions between
adjacent external cavity modes. For the top curve, the laser was
driven at 4.8 mW (97 mA) which is above the kink in the P-I graph.
In this regime, the features at 11.4 MHz and 22.8 MHz are clearly
visible indicating multiple external cavity modes. However, they
are still well below the power level of the primary mode.
Additional external cavity modes are visible up to several hundred
megahertz from the peak. The expanded spectrum at 11.4 MHz revealed
a 3 dB linewidth of several kHz, which supports the exceptional
observation of a main peak linewidth more narrow than the
resolution of the test equipment. A peak can also be seen at the
relaxation oscillation frequency of the laser of 8 Ghz. For the
conditions tested, the reflection sources is located well within
the coherence length of the laser. Therefore, in the stable P-I
regime, the laser is operating in a single external cavity mode.
With large amounts of back reflection and above the P-I kink,
several to many external cavity modes compete, although these
devices do not fall into complete coherence collapse. However, the
time averaged spectrum is effectively confined to several hundred
megahertz.
FIG. 12 shows the relative intensity noise spectrums with -36.5,
-22, -15, -8.6 and -7.0 dB of feedback into Module 4 operating at 1
mW. For very low back reflection levels, the intensity noise
measured is limited by the sensitivity of the test system. However,
at -22 dB of optical feedback, the relaxation oscillation and the
first harmonic is visible at 5.4 GHz. At the resonance peak, fine
oscillations spaced 11.4 MHz apart correspond to the mode spacing
of the external fiber cavity. For systems operating at 2.5 Gb/s,
low pass filters are used to eliminate wide band noise. Above -22
dB of optical feedback, the relaxation oscillation peak broadened,
and the RIN increased. In FIG. 13, the RIN at 1.8 Ghz is plotted
against the amount of feedback. With up to -20 dB back reflection,
the RIN at 1.8 Ghz was comparable to devices with isolators.
The low frequency intensity noise reported in the literature of
Fabry Perot diode lasers was not observed in FIG. 12. For Fabry
Perot laser diodes, the low frequency noise is attributed to power
drop events followed by multi-step return of power by individual
steps with an interval set by the round trip time of the external
cavity. The power increases and the spectral line decreases as the
FP laser settles into the state with the most narrow linewidth.
From FIG. 11, it can be seen that although the external cavity
modes are only 11.5 MHz apart, very few can be sustained under CW
operation. Since the grating limits multiple external cavity modes,
intensity noise from external cavity mode competition is also
avoided. Competition between modes can be promoted by increasing
the length of the external cavity to decrease the frequency between
modes. Preliminary data was taken with the back reflection source
set 10.5 km away from the laser. Under these conditions, the low
frequency RIN increased over the ground level by 10 dB with at a
drive current of 40 mA. Because of the increased fiber losses, the
power returning to the optical module was 4.3%.
The bit error rate of 2.5 Gb/s under direct modulation was taken
with varying amounts of optical feedback. The modules were tested
with an average fiber power of 1.1 mW, an extinction ratio of 8.0
dB, a psuedo-random bit stream (PRBS) of 2.sup.23 -1 and a chip
temperature of 25.degree. C. For transmission measurements over 100
km of SMF-28 fiber (dispersion D.about.17 ps/nm/km), the output
from the directly modulated laser and back reflection set was
boosted with an erbium-doped fiber amplifier (EDFA) to overcome
fiber and coupling losses in the system, and detected with an
avalanche photodiode (APD) receiver. FIG. 14 plots the error rate
of Module 4 as a function of receiver signal with the minimum and
maximum amount of feedback available with the test set-up. The
effect of the feedback-induced RIN can be seen from the change in
the 0 km receiver sensitivity. Measured at an error rate of
10.sup.-10 between the -36.5 dB and -7.0 dB back reflection
conditions, the change of 0.2 dB is near the resolution of the
test. The change in receiver sensitivity (.delta..sub.r) as a
function of the relative intensity noise and bandwidth (BW) is
given by:
For the group of modules tested, the observed degradation in 0 km
sensitivity was as large as 0.4 dB with back reflections of -7.0
dB. The Q factor for these modules without any feedback is
typically greater than 11 dB. These results are consistent with
Equation 1 and the measured RIN of -115 dB/Hz with maximum
feedback.
The dispersion penalty after 100 km of fiber is virtually unchanged
with the module operating under -7.0 dB of back reflection as shown
in FIG. 14. Even with large amounts of feedback, the side mode
suppression ration in FIG. 9 is greater than 52 dB so error floors
associated with sub-mode effects were avoided. The dispersion
penalty is plotted as a function of back reflection in FIG. 15. For
some modules, the dispersion penalty increases slightly under
extreme back reflection suggesting modes spectral broadening.
Equation 2 indicates the difference in propagation time for two
adjacent modes of a cavity with optical length n.sub.ext-cav
L.sub.ext-cav over a fiber with dispersion D and length L.sub.disp
:
With the 8.7 m fiber external cavity used here the propagation
delay after 100 km of fiber between adjacent modes will be less
than 0.1 pS. Under dynamic operation, multiple external cavity
modes may be excited. However, FIG. 11 suggests that the effective
broadening caused by these back reflection sources will be
contained by the grating spectral envelope. By minimizing
reflection sources less than 1 meter from the laser, jumps between
adjacent external cavity modes greater than 150 MHz can be avoided.
Such mode hops will be small compared to data modulation line
broadening. Close reflections can be minimized by coupling the
light from the laser into a fiber with an antireflection coating,
and setting a minimum length before a connector.
Semiconductor laser devices for high-speed data transmission
applications without the use optical isolators are disclosed. By
removing the isolator, the cost of producing these high-performance
optical devices is reduced. Also, fabrication of devices using
integrated optical combiners and other integrated optics is greatly
simplified, and easier integration of optical components within a
single module or on a monolithic chip is provided. The design and
assembly of multiwavelength sources in a single compact module, for
example, can be simplified if optical isolators are not required to
prevent cross-talk.
Also, DFB laser diodes monolithically integrated with
electroabsorption modulators usually require very good output faced
antireflection coatings to prevent light from coupling back into
the laser and generating undesirable chirp. However, the
antireflection coating requirements may be relaxed if the DFB laser
diode is resistant to the effects of optical feedback.
While this invention has been particularly shown and described with
references to preferred embodiments thereof, it will be understood
by those skilled in the art that various changes in form and
details may be made therein without departing from the scope of the
invention encompassed by the appended claims.
* * * * *